1. Field of the Invention
The present invention relates to the inspection of printed circuit boards and more particularly to a fixture facilitating the inspection of the underside of a printed circuit board after components have been inserted in the board but prior to their soldering in place.
2. Description of the Prior Art
In the manual assembly of printed circuit boards, unclinched electronic components are not fastened to the printed circuit board prior to wave soldering. In this mode of manufacture, only top side inspection of component placement for appropriate accuracy could be performed. The inspection of the bottom side of the printed circuit board would require turning the printed circuit board over which would then allow the components to fall out of the board due to the force of gravity. Obviously, the disadvantages of performing only top side inspection relates to the fact that if a component is mounted it may appear properly seated on the printed circuit board and it may then be assumed, and not always correctly, that all the component leads are protruding properly through the board on the solder side.
As noted, this assumption is not always correct. A condition sometimes exists where a component lead or leads may be bent under the component. Thus, while the component appears to be properly mounted when viewed from the top side, it may be incorrectly seated and thus when soldered will not function properly. This problem is an industrywide problem defined as "unseated components". The defect is especially harmful in that it may not be electrically detectable since contact between the bent lead or leads may be made on the top side of the printed circuit board although mechanical fastening via solder has not been achieved. Subsequent movement of the device, printed circuit board or the lead may cause an electrical failure of the final product prior to shipment or "in the field".
Prior solutions to this problem required a top side retainer to be installed to hold the components in place when the board is turned over for inspection of lead presence. The use of this technique obviously requires extra tooling, handling and frequently causes damage to fragile components. Yet another solution to the problem requires the spraying of an adhesive to the printed circuit board. This technique substantially increases material costs, as well as posing a risk to the electrical characteristics of the printed circuit and obviously is applicable only to very light components. In addition to the physical risks associated with these techniques, the techniques themselves produced marginal results since bottom side up inspection will not provide the necessary geographical and topographical contrast necessary to detect lead presence or absence.